Different methods are known for making unwarrented listening-in difficult in wireless telecommunication. One method involves so-called time scrambling, where a time interval of the signal is divided into blocks of a given length, which are then rearranged and transmitted. The character of human speech and the desire to obtain low apprehensibility in the distorted signal results in that the blocks cannot be made too short. Longer blocks would, on the other hand, result in troublesome delay. Trials have shown that the delay which is introduced by a scrambler with tolerably low apprehensibility of the distorted signal is just over one second, which is very irritating in duplex connections.
Another known method of distorting speech is frequency scrambling, where the signal frequencies are first divided into a number of bands. The bands are then rearranged, with some of them being reversed, to provide an unintelligible signal.
Frequency scrambling is a simple method in principle, but it necessitates expensive and voluminous filter equipment to transpose up a frequency band by modulation, carry out a band limitation and finally a down transposition.